The present invention relates to electrical grounding of shielded cables and wires to a connector housing, and more particularly to an improved backshell adapter with an easily removable interface yoke suitable for attaching cable shields and grounding leads through a low resistance path directly to the connector housing.
Electrical connectors coupled to a cable by way of a backshell adapter provide reliable means for attaching a cable to an electrical component. The backshell adapter secures the cable to the connector while the connector provides a releasable means for transmitting the cable signals across a junction. This arrangement is of particular value for sophisticated electronics equipment requiring multiple wires transmitting signals and power to external components. Such equipment is often found in aircraft, satellites, military vehicles, electronically controlled machinery, and similar equipment.
Wires in the cables of sensitive electronic equipment are susceptible to interference from electrical and magnetic sources both internal and external to a cable. As a means of shielding wires from such interference a braided metal sleeve or shield is often placed over individual wire, pairs of wires, and/or, and entire bundle of wires comprising a cable. The shield protects wires of the cable from electromagnetic interference (EMI) as long as they are suitably grounded and shielded at the ends of the cable. For cables attached to connectors, the metal housing of the connector provides the path to ground; the means for electrically grounding the wire shields to the housing being the subject of this disclosure.
Where used, a backshell adapter serves a variety of functions to mechanically and electrically transition the cable and wires to the connector. In the region just behind the connector, wires must be stripped and prepared for insertion into the connector. Sufficient wire must be stored to allow future repair access to connector pins, and grounding of external wire shields must be accommodated. The wires must be mechanically secured to the connector. Backshells provide an enclosed housing and mechanisms to support these functions.
The backshells are typically coupled to the connector by way of a mating interface with a threaded coupling that can be removed as needed for repair. The interface configuration and dimensions are supported by military and industrial standards to insure interchangability between components from different manufacturers. The interfaces are designed to insure reliable mechanical, environmental and electrical coupling. Electrically the interface must insure solid grounding and electrical and magnetic shielding with no windows for EMI leakage. Environmentally the backshell-to-connector interface must be sealed from external hazards such as corrosion inducing moisture. Mechanically the interface must maintain the electrical and environmental functions under tensile, bending and torsion loading, in addition to shock and vibration.
Connector and cable designs are available with a wide range of performance capabilities. Increasing performance comes with increased cost. Backshell adapters are typically chosen as a means of transitioning cables to connectors when the system cost favors repair over replacement for damaged assemblies. Backshell adapters are advantageous because they are designed to optimally transition a wide variety of cable configurations to a relatively few number of standard connector designs. This maintains connector cost lower by enabling a few designs to be produced in higher quantity. The backshell separates the transitional requirements from the electrical connection requirements, allowing each to be separately optimized for cost and performance.
There are several common methods used to terminate a wire shield to a connector. These methods including utilizing pigtails, jumpers, or grounding leads to create a ground path to a point on the backshell, or to a pin within the connector. These methods are time consuming to install or repair and are generally hampered by the lack of access to the wires within the backshell. In many instances, there is no adequate means within the structure to attach the ground leads to the backshell so as to be close to the mating connector interface. Leads are often secured at the rear of the backshell hardware, increasing the resistance of the ground path. Further, these methods rely on the conductivity of the backshell and are unsuitable for applications requiring a low conductivity, or non-conductive backshell material; examples include lightweight plastics or corrosion resistance stainless steel, not good conductors or shields. If the attachment of leads takes place outside of the protective backshell housing, then the termination is vulnerable to corrosion damage.
Methods using pigtails or ground leads often attach the leads at the point of mechanical coupling of the backshell to the cable. These methods require the ground leads to be disconnected every time the backshell is loosened to give access to the connector.
One method, for terminating shielded wires to a connector is presented by Van Brunt et al, U.S. Pat. No. 4,820,201. This method extensively modifies a standard connector to include a metal disk suitable for attaching wire shields by means of a transition clip secured over the shielded braid. The metal disk includes multiple openings arranged to coincide with the wire entry points commonly used on connectors. While providing a very short ground path for shields, this method requires an expensive connector modification, precluding the use of standard off the shelf connector configurations. Further each type of connector can have many insert arrangements, each with a different hole pattern. This arrangement requires a specific grounding disk be used for each insert arrangement, making the system prohibitively expensive for most applications.
Traditional ground leads methods have proven to be inadequate for simple effective grounding of shields. Further, methods that require extensive modification of connectors designs have been found to be prohibitively expensive. There exists a need for a cost effective method of grounding shields that insures a low resistance path to ground, and a reliable well protected termination that is both easy to install and repair.
The present invention provides for an improved backshell adapter and yoke configuration that readily accommodates the termination of ground leads secured to the braids of shielded wires and cables. In particular, it provides for a conductive removable connector interface with an attached means for terminating ground leads, said removable interface being referred to here as a yoke. The yoke is removably insertable into the front of a backshell designed to accept it. When the backshell is mated to a connector, the yoke mates directly to the connector interface providing a low resistance ground path to the connector housing for any lead, particularly ground braids, secured directly to the yoke.
The removable yoke provides many benefits to the user. First, it is a secured grounding platform for shield leads residing in close proximity to the connector housing. Multiple wire leads can be attached to the yoke, by any number of methods that are common in the industry, such methods including banding, soldering, or clipping to name a few. Second, the yoke provides ample access to the lead terminations during installation and repair. The ease of access to shield leads reduces the cost of terminating a cable to a connector. Third, the yoke allows the backshell to be removed from the connector to gain repair access to the wires without having to disconnect the grounding shield. This arrangement results in substantial time saving during repair or inspection, and eliminates one of the leading causes of cable problems; namely, failure to properly reground shield leads after decoupling a backshell.
As the connector interface for the backshell, the yoke must accommodate all of the mechanical, electrical and environmental interface requirements without adversely interfering with the form, fit or function of the connector. Further, to remain a viable design option, the yoke must conform to the dimensional limits of standardized connector and backshell specifications. As a final consideration, the yoke and mating backshell must be cost effective and must be capable of production by standard methods. All of these requirements dictate a very narrow envelope in which a yoke may be designed. Some connector interfaces do not permit a removable yoke owing to a conflict between two or more of the above requirements. The most commonly used circular connectors, however, including those governed by the military standards MIL-C-38999 and MIL-C-85049, enable a yoke design.
The yoke is a generally circular member having a maximum external diameter and a minimum internal diameter, both diameters set by the yoke""s functional requirements. Within the maximum and minimum envelope, the yoke design must accommodate ground lead termination, adapter body grounding, environmental sealing, anti-body rotation, and EMI shielding, as well as electrical field shielding. The maximum outside diameter of a removable yoke is limited by the minor dimension of the female threads on a coupling through which the yoke must be passed for removal. The minimum inside diameter is limited in some regions by the interface design standards, and in others the need to insert wires into the prearranged hole patterns of a connector insert. For each size connector, there is a minimum diameter needed to accommodate the largest possible wire configuration supported by the connector inserts. If the yoke ID is below the limit, it will not be functionally suitable for the connector.
Ideally, a connector and backshell interface would be designed with the needs of a removable yoke in mind. However, one of the important objects of the yoke is that it can function with the common connector interfaces that already enjoy an extensive installed user base.
One important feature of the yoke is that it can be made from a material different from the backshell adapter. The backshell can be constructed of a material best suited for its application, strength and adaptability to existing connectors while the yoke may be constructed of a conductive material best suited for grounding shield leads and providing electrical field and EMI shielding. In corrosive environments, such as marine atmospheres, a backshell body can be made of corrosion resistant stainless steel, while the yoke can be made of a much more conductive metal. Because the yoke is housed within the adapter, it will not see the harsh environment.
Yet another application for a yoke of dissimilar material is in the lightweight adapters for aircraft. A plastic body, possibly plated with a thin metal EMI coating, can be used for a lightweight adapter body. The yoke, being relatively small, can also be made of metal. In such an application, the heavier metal material is used only where needed for secure grounding.
The yoke is ideal for backshell adapters terminated to a flexible conduit. In such applications, a conduit with a pre-attached backshell is supplied to customers for wiring. In such instances, ground leads cannot be integrated with the adapter termination, leaving no effective means for grounding shield leads. The yoke provides a grounding platform, with all the advantages previously mentioned.
Many backshells are designed to secure a wire bundle by a mechanical strain relief structure that when loosened allows the adapter to slide down the cable giving ample repair access to the back of the connector. When a cable has internal shield terminations that are secured to the backshell, they must be disconnected before the adapter can be slid down the cable. When the yoke is used to terminate the ground leads, the adapter can be slid back down the cable while the shield leads stay securely terminated to the yoke. As the backshell adapter is slid down the cable, the yoke exists the front of the connector.
Another feature of the yoke is that it can be supplied with grounding leads pre-attached. When installed the leads can be grounded to a wire braid by a suitable solder method, several of which are common in the industry as a means for attaching leads to braid sleeves.
In addition to securing individual shield leads, the yoke can be used to ground the bulk shield of a cable. The bulk shield can be passed through the center of the adapter and secured circumferentially around the yoke termination area by one of several means, including a band, crimp or solder. This method has the advantage of allowing the yoke to provide EMI shielding right up to the back of the connector. Further, when decoupled from the connector, the backshell can be slid down the cable to give access to the back of the connector without having to disconnect the yoke. This feature is a significant advantage for repair.
It is an object of the present invention to provide a shield termination system that can be used with standard connectors.
It is an object of the present invention to provide a shield termination system that maintains a low resistance path to ground.
It is an object of the present invention to provide a shield termination system that protects terminations from the environments.
It is an object of the present invention to provide a shield termination system that allows repair access to the connector without disturbing the shield ground leads.
It is an object of the present invention to provide a shield termination system that allows the shields to be pulled clear of the backshell to provide working room for installation and repair.
It is an object of the present invention to provide a shield termination system that is lightweight.
It is an object of the present invention to provide a shield termination system that is resistant to the effects of vibration.
It is an object of the present invention to provide a low resistance shield termination system that allows a backshell to be constructed of less conductive material than previously while still providing electrical field and EMI shielding.
It is an object of the present invention to provide a shield termination system that will provide strain relief to individual wires.
It is an object of the present invention to provide a shield termination system capable of grounding a bulk cable braid.
It is an object of the present invention to provide a shield termination system that conforms to dimensional and performance requirements of industry standards for backshell interfaces.
The above and other features, objects and advantages of the present invention, together with the best means contemplated by the inventors thereof for carrying out the invention will become more apparent from reading the following description of a preferred embodiment and perusing the associated drawings in which: